1. Field of the Invention
The present invention relates to a communications system with moving bodies relayed by satellites. The Applicant has designated this system by the acronym "SYCOMORES" abbreviation for communications system with moving bodies relayed by satellites.
2. Description of Prior Art
The term moving body is understood to mean cars, lorries, ships, aircraft, trains and, by extension, any user equipped with portable terminals which are temporarily stationary.
In the early years of the space era communication satellites appeared with a period equal to 12 hours and with highly elliptical orbits (perigee approximately 1,000 km and apogee 39,000 km). These systems were mainly developed by the USSR with MOLNYA-type satellites.
However, this suffers from the following disadvantages: by no means negligible atmospheric deceleration due to the limited perigee height,
significant interference from the earth's potential, inversely proportional to the value of the semi-major axis of the orbit, double passage through the Van Allen regions,
needs to use at least three satellites for a 24 hour coverage, elongated lenticular shape of the coverage zone in accordance with an east-west axis, not very suitable for Western Europe, significant variation in the satellite-earth distance leading to a high Doppler effect and a "zoom" effect causing radio coverage and attitude control problems.
The event of geostationary satellites made it possible to solve certain of these problems. Such satellites, with their equatorial circular orbit and period of 24 hours, seem to be immobilized above the earth, so that they can constitute privileged relays for telecommunications networks.
However, this solution still suffers from disadvantages. Thus, the elevation angle of a geostationary satellite, seen from a moving body on the ground, varies with the latitude of the moving body, which implies a pointing of the antennas towards the satellite. The following table gives values for the elevation as a function of the latitude of a place, for a certain number of towns in the northern hemisphere in an area between the Maghreb and Scandinavia.
______________________________________ LATITUDE MAX. ELEVATION ______________________________________ KIRUNA 68.degree. 16.degree. 2 STOCKHOLM 59.degree. 3 23.degree. COPENHAGEN 55.degree. 5 26.degree. 5 AMSTERDAM 52.degree. 3 30.degree. PARIS 49.degree. 1 33.degree. 8 TURIN 45.degree. 2 38.degree. 5 MADRID 40.degree. 6 44.degree. 6 ALGIERS 36.degree. 6 47.degree. 3 ______________________________________
Although this elevation varies, it is relatively small for high latitudes, which causes transmission problems. Thus, for an elevation angle of e.g. 38.degree., a 8 m high obstacle 10 m from a moving body would prevent communication.
However, such obstacles are frequently encountered in cities and suburbs.
Moreover, the azimuth of the moving body-satellite direction changes continuously with respect to the moving body, when the latter changes its direction of movement. It is therefore necessary to provide means for permanently pointing the antenna of the moving body towards the satellite. Finally, with regards to the retransmission means on board the satellite, their power being proportional to 1/sin E (in which E is the elevation), it varies with the latitude of the working area, or at least it is fixed to a high value dictated by the lowest elevation.
In order to obviate these disadvantages, constellations of satellites have been proposed with coordinated orbits such that a permanent coverage is ensured on the ground. For example, in the GPS/NAVSTAR system, there are 21 satellites with circular orbits of period 12 hours, four such satellites always being in direct view from a point on the earth. European patent application 213 355 describes a constellation of this type, but with only four satellites having elliptical orbits. These satellites have the same period and different traces on the ground. Two satellites have their perigee in the northern hemisphere and the two others in the southern hemisphere. Therefore any point on earth can "see" one of these satellites at all times. However, such a system still suffers from disadvantages, in the sense that the elevation of the satellite seen from the moving body varies considerably between individual regions of the earth.
Other constellations are described in the article by J. E. DRAIM entitled "Three- or four-satellite continuous-coverage constellations" published in "Journal of Guidance, Control and Dynamics", vol. 8, No. 6, November/December 1985, pp 725-730.
Although of interest in certain respects, none of these constellations makes it possible to solve all the problems inherent in communications with moving bodies, which are essentially as follows: first it is highly desirable to get round the need from having to orient the vehicle antenna towards the satellite; it is preferable to be able to use high gain antennas (e.g. above 10 dB) in order to increase the transmission capacity; finally, the system must be well protected against interference from the environment.